ixgb: fix unload race with timers

ixgb needs to call flush scheduled work to flush any timers before
unregistering the netdev.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>